This invention relates generally to a method of making thermosetting composites and the resulting product which preferably comprises electrical circuit laminate materials. More particularly, this invention relates to an electrical circuit laminate comprising a thermosetting resin of polybutadiene or polyisoprene which is subjected to a high temperature cure at a temperature of greater than about 250xc2x0 C.; and which may optionally include (1) a second polymer which cross-links with the polybutadiene or polyisoprene resin; (2) a woven fibrous web impregnated with the resin; and (3) optional inorganic particulate filler such as silica, titania and the like.
Commonly assigned U.S. Pat. No. 5,223,568 (which is fully incorporated herein by reference) describes a thermosetting composition which is particularly useful for making electrical substrate materials. In general, U.S. Pat. No. 5,223,568 describes a composition formed from the steps of:
(a) providing a moldable thermosetting composition that includes 1) polybutadiene or polyisoprene resin which is a liquid at room temperature and which has a molecular weight less than 5,000 and 2) a solid butadiene or isoprene-containing polymer capable of cross-linking with the polybutadiene or polyisoprene resin;
(b) forming the composition into a shape; and
(c) curing the composition to produce the electrical substrate material including subjecting the composition to a high temperature cure condition at a temperature greater than about 250xc2x0 C. and less than the decomposition temperature of the composition. This composition thus comprises a two component system, the first component being the polybutadiene or polyisoprene resin and the second component being the solid butadiene or isoprene-containing polymer; all of which are subjected to the high temperature curing cycle (e.g., greater than 250xc2x0 C.).
In preferred embodiments, the solid polymer is a thermoplastic elastomer block copolymer.
U.S. Pat. No. 5,223,568 also describes a composition with a dielectric filler (i.e., a material having a dielectric constant greater than about 1.2 at microwave frequencies) homogeneously dispersed throughout the composition to the extent that when the composition is cured the properties of the cured article, e.g., dielectric constant and coefficient of thermal expansion, do not vary more than about 5% throughout the article.
In preferred embodiments, the composition of U.S. Pat. No. 5,223,568 further includes a crosslinking agent capable of co-curing (i.e., forming covalent bonds) with the polybutadiene or polyisoprene resin thermoplastic elastomer, or both. Examples of preferred crosslinking agents include triallylcyanurate, diallylphthlate, divinyl benzene, a multifunctional acrylate, or combinations of these agents.
When the electrical substrate material includes a dielectric filler, the volume % of the filler (based upon the combined volume of resin, thermoplastic elastomer, crosslinking agent (if any) and filler) is between 5 and 80%, inclusive. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres); corundum, wollastonite, polytetrafluoroethylene, aramide fibers (e.g., Kevlar), fiberglass, Ba2Ti9O20, glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, or magnesia. They may be used alone or in combination.
The method disclosed in U.S. Pat. No. 5,223,568 provides a wide variety of shaped articles having favorable isotropic thermal and dielectric properties. These properties can be tailored to match or complement those of ceramic materials, including Gallium Arsenide, alumina, and silica. Thus, the cured articles can replace ceramic materials in many electronic and microwave applications, e.g., as specialized substrates for high speed digital and microwave circuits. Examples of microwave circuits including microstrip circuits, microstrip antennas, and stripline circuits. The cured products are also useful as rod antennas and chip carriers.
Circuit laminate materials of the type described in U.S. Pat. No. 5,223,568 are commercially available from Rogers Corporation, Rogers, Connecticut and sold under the trademark xe2x80x9cTMMxe2x80x9d. Such circuit materials may be produced by initially casting a slurry of resin and filter followed by lamination (e.g., molding) and the high temperature cure cycle. While laminates produced by using an initial casting step exhibit excellent cost efficiencies and electrical properties, it has been determined that such materials do suffer from poor dimensional stability and exhibit undesirable brittleness. It has also been determined that the two component polybutadiene or polyisoprene based thermosetting composition described in U.S. Pat. No. 5,223,568 may not be desirable for all anticipated applications of the material, particularly when used as a circuit laminate material.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the electrical substrate material of the present invention. In accordance with thee present invention, it has been discovered that:
(1) in a two component system, the first component (e.g., polybutadiene or polyisoprene resin) is not required to (a) be liquid at room temperature, (b) have a molecular weight less than 5,000, or (c) have pendent vinyl groups nor is the second component (e.g., the unsaturated butadienexe2x80x94or isoprene-containing polymer) required to be a xe2x80x9csolidxe2x80x9d polymer;
(2) the thermosetting composition need not be a two component system; but instead may be comprised of a single polybutadiene or polyisoprene containing resin component (e.g., without the butadiene or isoprene-containing polymer) or the single resin component may comprise a polybutadiene or polyisoprene containing polymer such as SBS triblock polymers;
(3) woven or nonwoven fibrous webs may be incorporated into dielectric resins of the type described in U.S. Pat. No. 5,223,568 and in the resins systems of (1) and (2) above whereupon the deficiencies in the prior art related to undesirable dimensional stability and brittleness are dramatically improved with only a relatively small loss in electrical performance. In addition, the resulting electrical laminate may be produced at a relatively low cost thus permitting the use of the present invention in high volume applications, particularly high volume microwave applications and those uses requiring thin, low cost laminates.
The foregoing discovery is premised, at least in part, on the understanding that a critical part of the invention in U.S. Pat. No. 5,223,568 is heating the thermosetting material to a temperature greater than about 250xc2x0 C. This high temperature cure step is unconventional for diene polymers such as polybutadiene or polyisoprene resins, with or without the presence of usual or other curatives such as peroxide or sulfur. Thus, it is the high temperature curing step of a polybutadiene or polyisoprene resin that defines this invention. The use of (a) a two component system, (b) a liquid resin, (c) a resin having a MW less than 5,000, (d); pendent vinyl groups, or (e) a solid butadiene or isoprene polymer are not required or even desirable under certain circumstances. The process of this invention thus comprises:
Two Component System
A forming process for producing an article such as an electrical circuit substrate comprising the steps of:
(a) providing a thermosetting composition comprising;
a polybutadiene or polyisoprene resin; and
an unsaturated polybutadiene or polyisoprene containing polymer capable of participating in cross-linking with said polybutadiene or polyisoprene resin during cure; and
(b) curing said composition to produce said article (e.g., electrical circuit substrate) by subjecting said composition to a high temperature cure condition at a temperature greater than about 250xc2x0 C. and less than the decomposition temperature of said composition.
One Component System
A forming process for producing an article such as an electrical circuit substrate comprising the steps of:
(a) providing a thermosetting composition comprising a polybutadiene or polyisoprene resin; and
(b) curing said composition to produce said article (e.g., electrical circuit substrate) by subjecting said composition to a high temperature cure condition at a temperature greater than abut 250xc2x0 C. and less than the decomposition temperature of said composition.
The electrical substrate material of this invention may also include a plurality of woven webs (such as E-glass webs) embedded in a mixture of the polybutadiene or polyisoprene based resin system and optional inorganic filler (e.g., silica) laminated between one or two sheets of conductive foils (e.g., copper) to produce a circuit board material which is especially well suited for microwave applications. Of course, if very thin (e.g., less than 5 mil thickness) cross-sections are desired, then only a single saturated web may be used for the dielectric.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those or ordinary skill in the art from the following detailed description and drawings.